The intensive use of antibiotics has exerted a selective evolutionary pressure on micro-organisms to produce genetically based resistance mechanisms. Modern medicine and socio-economic behaviour exacerbates the problem of resistance development by creating slow growth situations for pathogenic microbes, e.g. in artificial joints, and by supporting long-term host reservoirs, e.g. in immuno-compromised patients.
In hospital settings, an increasing number of strains of Staphylococcus aureus, Streptococcus pneumoniae, Enterococcus spp., and Pseudomonas aeruginosa, major sources of infections, are becoming multi-drug resistant and therefore difficult if not impossible to treat:                S. aureus is resistant to β-lactam, quinolone and now even to vancomycin;        S. pneumoniae is becoming resistant to penicillin, quinolone and even to new macrolides;        Enterococci are quinolone and vancomycin resistant and β-lactams are inefficacious against these strains;        Enterobacteriacea are cephalosporin and quinolone resistant;        P. aeruginosa are β-lactam and quinolone resistant.        
Further new emerging organisms like Acinetobacter which have been selected during therapy with the currently used antibiotics are becoming a real problem in hospital settings.
In addition, microorganisms that are causing persistent infections are increasingly being recognized as causative agents or cofactors of severe chronic diseases like peptic ulcers or heart diseases.